In recent years, batteries such as lithium ion secondary batteries are utilized in various fields, e.g., electronic devices such as a cell phone and a personal computer, and vehicles such as a hybrid vehicle and an electric vehicle. The lithium ion secondary battery particularly is high in energy density and therefore suitable to be mounted in various devices.
One example of the structure of the above secondary batteries is known as one disclosed in Patent Document 1 listed below. The secondary battery disclosed in Patent Document 1 is configured such that a wound electrode body (a flat wound electrode body 4) is housed in a rectangular battery case (a case 3; names and reference signs in Patent Document 1 are parenthesized in the following explanation). The wound electrode body includes a positive electrode sheet (a positive electrode sheet 41) formed of a foil-shaped positive current collecting part (a positive current collecting part 413) having positive active material layers (positive active material layers 412) on surfaces, a negative electrode sheet (a negative electrode sheet 42) formed of a foil-shaped negative current collecting part (a negative current collecting part 423) having negative active material layers (negative active material layers 422) on surfaces, and separators (separators 43) insulating the positive electrode sheet and the negative electrode sheet, which are wound together into a flattened shape. This wound electrode body has a wound positive end part (a protruding end part 411) formed of a positive active material layer unformed portion (an edge portion 414) formed with no positive active material layer, the positive end part being located on one end side in a winding axis direction and also has a wound negative end part (a protruding end part 421) formed of a negative active material layer unformed portion (an edge portion 424) formed with no negative active material layer, the negative end part being located on the other end side in the winding axis direction.
The positive end part is joined with a positive terminal member (a positive terminal 1) and the negative end part is joined with a negative terminal member (a negative terminal 2). A junction part with the positive terminal or the negative terminal in each end part (each protruding end portion 411, 421) is formed of the laminated positive active material layer unformed portion (the edge portion 414) or the laminated negative active material layer formed unformed portion (the edge portion 424) compressed in a thickness direction of a battery case perpendicular to the winding axis direction (see FIG. 1 and [0040] of Patent Document 1). In the secondary battery disclosed in this document, particularly, a part of each end part (each protruding end portion 411, 421) is completely crushed (compressed) in the thickness direction of the battery case when viewed from an end face thereof in the winding axis direction.
The above configured secondary battery provides a difference in thickness between both end parts (the protruding end portions 411, 421) in the winding axis direction of the wound electrode body and a middle portion present between the end parts. Accordingly, the space generated by this thickness difference can receive the positive terminal member and the negative terminal member. This achieves size reduction of the entire battery (see [0018] of Patent Document 1).